The 45° lock miter joint is considered by many woodworkers to be one of the most difficult to make. Proper tools, like a router table with a large work surface, stiff fence, and a powerful router positioned beneath the work surface, are a necessity. Furthermore, boards being worked must be flat and well prepared to prevent exit tear-out and chipping. Finally, a high degree of skill in adjusting the height and depth of cut of a lock miter bit is needed since a lock miter joint must cut in a single pass.
Set-up of a router table for making lock miter joints is time-consuming. In order to produce matching joints, the router is moved up and down and the fence is moved back and forth to adjust the height and depth of a cut. Unfortunately, any movement of the router to adjust cutting height has an effect on both cutting depth and height and the same is true with movements of the fence.
Those brave enough to make lock miter joints are rewarded with woodwork that is beautiful and strong. Lock miter joints, as is well known, are virtually invisible and cannot be seen from the outside of an object constructed with them. Further, such joints are sturdy since they have a large surface for gluing.